1. Field of the Invention
The present invention relates to a coil-integrated switching power supply module including a transformer including a power transfer-voltage conversion function between a primary side and a secondary side.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2000-260639, Japanese Unexamined Patent Application Publication No. 2004-349293, and Japanese Patent Application No. 3196187 each disclose a sheet transformer and an insulated switching power supply using the sheet transformer. The sheet transformer has a structure in which the wiring patterns of a primary wiring and a secondary wiring are formed on multiple substrates, through holes are provided in a multilayer substrate having the multiple substrates stacked therein, and the multilayer substrate is sandwiched with a ferrite core from both sides.
FIG. 1 and FIG. 2 of the present application correspond to FIG. 3 and FIG. 4 of Japanese Patent Application No. 3196187. FIG. 1 is an equivalent circuit diagram of an insulated direct current (DC)-DC converter. FIG. 2 is a perspective view showing the structure of the circuit in FIG. 1, which describes how the circuit is mounted. The core of a transformer T is mounted at a central portion of a wiring layer 20, a switching element Q1, such as a field effect transistor (FET) or a transistor, is provided at a primary side of the transformer T, and diodes D3 and D4 and the core member of a choke coil L1 are provided at a secondary side of the transformer T. A shield layer 30 is provided immediately below the wiring layer 20 and a coil inner wiring layer 12 is provided below the shield layer 30.
Wiring patterns between inter-layer connection holes of the wiring layer 20 to a coil layer 10 and a capacitor layer are provided in the coil inner wiring layer 12, and the wiring layer 20 and the coil inner wiring layer 12 compose two wiring areas.
The coil layer 10 is provided below the coil inner wiring layer 12. A primary winding n1 and a secondary winding n2 of the transformer T are stacked in the coil layer 10, and the coil layer 10 is positioned so as to be magnetically affected by the core T mounted on the wiring layer 20. In addition, a wiring n3 for the choke coil L1 is also provided in the coil layer 10 at a position corresponding to the position where the core member of the choke coil L1 is arranged. The capacitor layer 40 is provided below the coil inner wiring layer 12 and an output capacitor C2 is provided in the capacitor layer 40.
Japanese Patent Application No. 3196187 differs from Japanese Unexamined Patent Application Publication No. 2000-260639 and Japanese Unexamined Patent Application Publication No. 2004-349293 in that a coil pattern composing the transformer is provided in the multilayer substrate and only electronic components, the ferrite core, etc. are mounted on the surface layer. Since the wiring layer and the shield layer are required to be provided outside the coil pattern layer in such a structure, at least four layers including the surface-side layers and bottom-side layers are not used for the coils. Accordingly, it is necessary to increase the number of layers of the substrate in order to set the conductive resistance of the coils to a sufficiently low value. However, the increase in the number of layers of the substrate causes a problem in that the manufacturing cost of the substrate is greatly increased. In addition, there is a problem in that the thickness of the substrate is also increased in association with the increase in the number of layers to increase the thickness of the product. In contrast, it is relatively easy to provide a measure against a leakage flux from the transformer and/or the choke coil by not arranging a circuit having a low noise tolerance in a region where the leakage flux remarkably occurs. Accordingly, the structure in which the coil patterns are formed also on the surface of the substrate, as shown in Japanese Unexamined Patent Application Publication No. 2000-260639 and Japanese Unexamined Patent Application Publication No. 2004-349293, is more common.
Coil-integrated switching power supply modules in the related art each including a multilayer substrate in which spiral coil conductor patterns are formed on multiple layers and a magnetic core that extends through the multilayer substrate to compose a closed magnetic circuit have the following problems.
The multilayer substrate in which spiral coil conductor patterns are formed on multiple layers and the magnetic core that extends through the multilayer substrate to compose a closed magnetic circuit are used to form a transformer or an inductor in the coil-integrated switching power supply module. However, when the transformer or the inductor processes high power, large current flows through the spiral coil conductor patterns to make heat generation in the coil conductor patterns remarkable. The heat generated in the coil conductor patterns is transferred in a direction toward the periphery of the magnetic core and the multilayer substrate and is radiated outside from the magnetic core, the multilayer substrate, input-output terminals of the coil-integrated switching power supply module, and so on.
One object of the coil-integrated switching power supply module is to reduce the number of components and reduce the size of the coil-integrated switching power supply module and the size of the multilayer substrate is also preferred to be reduced as much as possible. Accordingly, heat concentration is likely to occur in the coil conductor patterns in association with the reduction in size of the entire coil-integrated switching power supply module. If the upper temperature limit of a material of the multilayer substrate is exceeded because of the heat concentration in the coil conductor patterns, color change and/or deformation of the multilayer substrate are caused thereby greatly degrading reliability. In addition, the components mounted near the coil conductor patterns are also adversely affected. When it is not possible to mount a fan for cooling the coil-integrated switching power supply module or a case in which the coil-integrated switching power supply module is housed or when the environmental temperature is high, the problem of the heat radiation gets worse.